The chrome material is conventionally etched with an aqueous composition comprising a cerium (IV) salt such as ammonium cerium (IV) nitrate [(NH.sub.4).sub.2 Ce(NO.sub.3).sub.6 ] and cerium (IV) sulfate [Ce(SO.sub.4).sub.2 ] and optionally perchloric acid, acetic acid, nitric acid and the like. Ratios and concentrations of these components depend on properties of the chrome material such as the oxidation degree of chromium, contents of impurities and a thickness of the chrome material as well as an etching rate. However, such conventional etchant composition is not suitable for use in etching a pattern with very thin gaps.
For example, a chrome mask, which is used for transferring a specific pattern onto a silicon wafer in the production of semiconductor integrated circuit elements, is produced by forming a chrome thin layer on a substrate made of a material which is transparent to visible light, ultraviolet light or far-infrared light to be used for transferring the pattern onto the wafer (e.g. glass and silica) followed by application of a photosensitive agent on the chrome thin layer to form a resist layer, then providing a latent image on the resist layer by irradiation of ultraviolet light, X-ray, an electron beam and the like, developing a designed pattern in the resist layer. Then, the chrome thin layer is etched through the pattern-formed resist layer as a mask so as to transfer the pattern to the chrome layer
As the degree of integration of the semiconductor integrated circuit increases, line gaps in the image to be transferred become thinner and thinner. In such circumstance, the conventionally used etchant can hardly penetrate into the thinner gaps formed in the resist layer so that some parts of the chrome layer are not etched.
To overcome such defect of the conventional etchant, there are proposed to add a pretreating step comprising dipping the resist in an organic solvent such as alcohol or an aqueous solution of a surfactant to improve the wettability of the resist (cf. Japanese Patent Kokai Publication (unexamined) No. 62854/1984) or to add an operation comprising vibrating the etchant with ultrasonic wave or agitating the etchant to facilitate the penetration of the etchant into the very thin gaps.
These proposals, however, are not favorable since they increase the number of steps of the etching process, or the etching quality is influenced by the fluctuation of concentration of the etchant. Further, the precisely formed resist image may be damaged, the chrome layer may drop out, or flaws are formed on the chrome layer.